1. Field of the Invention
The present invention relates to a discharge lamp driving apparatus for lighting a discharge lamp to illuminate a liquid crystal display (LCD) apparatus, and more specifically to a discharge lamp driving apparatus for lighting multiple discharge lamps.
2. Description of the Related Art
The LCD apparatus is one of flat panel display apparatuses, and is extensively used. Since a liquid crystal used in the LCD apparatus does not emit light by itself, a lighting device is required for ensuring a good screen display. A backlight system is one of such lighting devices, and illuminates the liquid crystal from behind. The backlight system uses mainly a cold cathode fluorescent lamp (CCFL) as a discharge lamp, and is provided with a discharge lamp driving apparatus including an inverter to drive the CCFL.
Since the LCD apparatus is increasingly getting larger and larger in size to meet applications to, for example, a large TV, the backlight system uses multiple discharge lamps for achieving sufficient illumination intensity over the screen of the LCD apparatus. The discharge lamps are each required to emit highly luminous light with uniform luminance among them. Variation in luminance among the discharge lamps causes uneven brightness over the screen of the LCD apparatus, which raises display and visual problems thus significantly deteriorating the product quality. Also, to answer a demand for a reduced cost on the LCD apparatus, cost reduction on the discharge lamp driving apparatus incorporated in the backlight system is strongly requested.
Variation in luminance of the discharge lamps can be reduced by equalizing lamp currents flowing therein. The equalization is enabled by providing transformers in a number corresponding to the number of the discharge lamps and controlling the transformers by a control IC. This approach, however, involves an increase of components, and pushes up the cost on the discharge lamp driving apparatus. An alternative approach for enabling the equalization of lamp currents is proposed which is accomplished by providing balance coils, but the alternative approach must use a large number of balance coils for multiple discharge lamps, and to make matters worse the balance coils must come up with individually different specifications due to the lamp currents differing depending on the places where they are disposed. Consequently, the number of components is increased pushing up the cost on the discharge lamp driving apparatus.
A discharge lamp driving apparatus as another approach is proposed, in which inductance values are controlled by variable inductance elements, rather than balance coils, so as to control respective lamp currents for uniform brightness over the display screen (refer to, for example, Japanese Patent Application Laid-Open No. H11-260580).
FIG. 1 is a block diagram of a discharge lamp driving apparatus which is disclosed in the aforementioned Japanese Patent Application Laid-Open No. H11-260580, and in which two discharge lamps are provided.
Referring to FIG. 1, FET's 102 and 103 constituting switching elements are connected in series between the positive and negative electrodes of a DC power supply 101, and the connection midpoint between the source terminal of the FET 102 and the drain terminal of the FET 103 is connected to the negative electrode of the DC power supply 101 via a series resonant circuit 120A which consists of a capacitor 122a and a coil 121a of an orthogonal transformer 121A which constitutes an variable inductance capable of controlling inductance value, and also via a series resonant circuit 120B which consists of a capacitor 122b and a coil 121a of an orthogonal transformer 121B which constitutes an variable inductance.
A connection midpoint between the coil 121a of the orthogonal transformer 121A and the capacitor 122a is connected to the negative electrode of the DC power supply 101 via a series circuit consisting of a capacitor 110a, a discharge lamp 111a, and a current detecting resistor 123a of a control circuit 123A, and an output signal of the control circuit 123A is sent to a control coil 121b of the orthogonal transformer 121A.
The control circuit 123A supplies a control current to the control coil 121b of the orthogonal transformer 121A, and is arranged such that a connection midpoint between the discharge lamp 111a and the current detecting resistor 123a is connected to the inverting input terminal of an operation amplification circuit 123c via a rectifying diode 123b, a connection midpoint between the rectifying diode 123b and the inverting input terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via a smoothing capacitor 123d, the non-inverting terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via a battery 123e having a reference voltage Vref to determine a reference value of a current of the discharge lamp 111a, and that the output terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via the control coil 121b of the orthogonal transformer 121A.
The control circuit 123A functions to control the current of the discharge lamp 111a. Specifically, the control circuit 123A operates such that, when the current of the discharge lamp 111a is to be increased, the control current of the control coil 121b of the orthogonal transformer 121A is increased so as to decrease the inductance value of the coil 121a of the orthogonal transformer 121A thereby increasing the resonant frequency f0 of the series resonant circuit 120A thus decreasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in an increase of a voltage generated between the both ends of the capacitor 122a, and such that, when the current of the discharge lamp 111a is to be decreased, the control current of the control coil 121b of the orthogonal transformer 121A is decreased so as to increase the inductance value of the coil 121a of the orthogonal transformer 121A thereby decreasing the resonant frequency f0 of the series resonant circuit 120A thus increasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in a decrease of a voltage generated between the both ends of the capacitor 122a. 
There is provided another circuit which includes another orthogonal transformer 121B, and which is constituted same as the above-described circuit including the orthogonal transformer 121A. Specifically, a connection midpoint between the coil 121a of the orthogonal transformer 121B and the capacitor 122b is connected to the negative electrode of the DC power supply 101 via a series circuit consisting of a capacitor 110b, a discharge lamp 111b, and a current detecting resistor 123a of a control circuit 123B, and an output signal of the control circuit 123B is sent to a control coil 121b of the orthogonal transformer 121B.
The control circuit 123B supplies a control current to the control coil 121b of the orthogonal transformer 121B, and is arranged such that a connection midpoint between the discharge lamp 111b and the current detecting resistor 123a is connected to the inverting input terminal of an operation amplification circuit 123c via a rectifying diode 123b, a connection midpoint between the rectifying diode 123b and the inverting input terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via a smoothing capacitor 123d, the non-inverting terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via a battery 123e having a reference voltage Vref to determine a reference value of a current of the discharge lamp 111a, and that the output terminal of the operation amplification circuit 123c is connected to the negative electrode of the DC power supply 101 via the control coil 121b of the orthogonal transformer 121B.
The control circuit 123B functions to control the current of the discharge lamp 111b. Specifically, the control circuit 123B operates such that, when the current of the discharge lamp 111b is to be increased, the control current of the control coil 121b of the orthogonal transformer 121B is increased so as to decrease the inductance value of the coil 121a of the orthogonal transformer 121B thereby increasing the resonant frequency f0 of the series resonant circuit 120B thus decreasing the impedance of the series resonant circuit 120B at a driving frequency consequently resulting in an increase of a voltage generated across the both ends of the capacitor 122b, and such that, when the current of the discharge lamp 111b is to be decreased, the control current of the control coil 121b of the orthogonal transformer 121B is decreased so as to increase the inductance value of the coil 121a of the orthogonal transformer 121B thereby decreasing the resonant frequency f0 of the series resonant circuit 120B thus increasing the impedance of the series resonant circuit 120B at a driving frequency consequently resulting in a decrease of a voltage generated across the both ends of the capacitor 122b. 
Also, in the discharge lamp driving apparatus shown in FIG. 1, a control circuit 104 fixedly sets a switching frequency of a control signal to be supplied to the FET's 102 and 103 whereby the currents flowing in the discharge lamps 111a and 111b are controlled at a predetermined value without controlling the switching frequency, thus allowing the circuit to be structured without complicated frequency control performed at the control circuit 104, and achieving uniform brightness between the discharge lamps 111a and 111b. 
Depending on the specifications of CCFL'S, a voltage to turn on the CCFL is generally higher than a voltage to keep it lighted. Specifically, the voltage to turn on the CCFL ranges from about 1,500 to 2,500 V while the voltage to keep it lighted ranges from about 600 to 1,300 V. Accordingly, a high-voltage power supply is required in a discharge lamp driving apparatus.
Since the discharge lamp driving apparatus shown in FIG. 1 is not provided with a step-up circuit, the DC power supply 101 has a circuitry to output a high voltage in order to duly drive the discharge lamps 111a and 111b. 
Also, since the FET's 102 and 103 to turn on the discharge lamps 111a and 111b, and the control circuit 104 to control the FET's 102 and 103 are connected to the DC power supply 101 to output a high voltage, the FET's 102 and 103 and the control circuit 104 must be composed of high-voltage-resistant materials which are expensive thus pushing up the cost of the apparatus.
Further, in the discharge lamp driving apparatus shown in FIG. 1, the capacitors 110a and 110b, which are current controlling capacitors (so-called “ballast capacitors”) to stabilize the lamp current of the discharge lamps 111a and 111b, are connected in series to the discharge lamps 111a and 111b, respectively, and a high voltage is applied to the capacitors 110a and 110b. Consequently, the capacitors 110a and 110b must also be composed of high-voltage-resistant materials, and since the current controlling capacitors must be provided in a number equal to the number of discharge lamps to be driven, the cost of the apparatus is pushed up definitely. Also, since a high voltage is applied to the capacitors 110a and 110b as described above, there is a problem also in terms of component safety.